There was a nice little Bonus interview on the "Wordplay" movie DVD with Manny Nosowsky and Joe DiPietro about constructing the first 19-block puzzle. I think Kevin Der said that was his inspiration for constructing his 18-block puzzle. There's a nice interview with Kevin on JimH's old blog: http://www.xwordblog.com/2008/08/kevin-g-der-the-new-champion.html

I'll just reemphasize a point I made in that interview that the key to achieving clean fill is to be diligent about reranking one's wordlist. In fact, even if you have a messy wordlist, you can still clean up the portion that's needed for any puzzle you are currently working on: If you're using Crossword Compiler in manual mode, word-selection menus will come up for each position in the grid: just right-click on any word in the menu, and a banner will come up which will let you change the rank. After you change a few word ranks in your menu, just click on the "Score" header and that menu will be resorted with the new ranks. (Alas, I stray, but hopefully it's a useful tip).

I'm not familiar with the CrossFire software; I've always used Crossword Compiler to construct puzzles, but the principle behind grid-filling software is probably the same: that the constructor pre-assigns a number -- usually from 1 to 99 -- to each word in the word list. Then, when the software is invoked to fill a grid, it will first try the words with the highest ranks (or word score) and work it's way down to lower ranks until the first successful fill is achieved.

Perhaps someone familiar with CrossFire might chime in to elaborate on those other scores you mention; they don't appear to be universal.-Joe